1. Field of the Invention
The invention relates generally to a wireless communication system, like a cellular network, and more particularly to downlink transmission of a reference signal in the network.
2. Description of the Related Art
A cellular network is a wireless communication system made up of a number of cells, each served by a fixed transmitter, known as a cell site or base station. Each cell site in the network typically overlaps other cell sites. The most common form of cellular network is a mobile phone (cell phone) system. All of the base stations are connected to cellular telephone exchanges or “switches”, which in turn connect to the public telephone network or another switch of the cellular company.
The 3rd Generation Partnership Project (3GPP) is a worldwide consortium to create a specification for a globally applicable third generation (3G) mobile phone system. 3GPP's plans are currently in development under the title Long Term Evolution (LTE). The 3GPP LTE project is to improve the Universal Mobile Telecommunications System (UMTS) terrestrial radio access (UTRA) mobile phone standard to cope with future requirements. Goals of 3GPP LTE include improving efficiency, lowering costs, improving services, making use of new spectrum opportunities, and better integration with other open standards. The evolved UTRA (E-UTRA) system proposed by 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) for the downlink (base station to mobile phone or terminal) and single carrier frequency division multiple access (SC-FDMA) for the uplink (mobile terminal to base station). In 3GPP LTE terminology, a base station is called a “Node-B” and a mobile terminal is called “user equipment” (UE). The 3GPP LTE technical specification is described in the reference document titled 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Channels and Modulation (Release 8), 3GPP TS 36.211 V0.4.0 (2007-02).
The 3GPP LTE proposes a multiple-input multiple-output (MIMO) system with up to four antennas per base station. This requires that at least some of the terminals in the network be capable of receiving not only base station transmissions from two antennas (2-TX) but also base station transmissions from four antennas (4-TX). The base stations transmit a downlink reference signal (DLRS) that is modulated into reference symbols that are used by the terminals for channel estimation and measurements. There are two reference symbol structures of interest to 3GPP LTE: one for a 2-TX base station and one for a 4-TX base station. Thus each base station may be transmitting both reference symbol structures in each downlink radio frame and at the same location in each radio frame.
It is desirable to minimize the overhead occupied by the DLRS in the downlink. Also, the transmission by adjacent Node-Bs of the DLRS may cause multiple access interference (MAI) which will degrade performance. Thus what is needed is a 3GPP LTE wireless communication system that optimizes transmission of the DLRS from an individual Node-B as well as from adjacent Node-Bs.